Today, printers utilizing optical modulators, including laser printers utilizing electrophotographic technology, are widely used as printers connected to personal computers and networks, and also as digital copying machines, digital printers, color copying machines, color printers, or the like. The printers of this type are called optical printers, since in each printer, image formation is controlled by controlling pixel units for forming characters and images by ON/OFF control of optical power.
As means for the ON/OFF control of the optical power in the optical printer, a laser diode array or a light emitted diode (LED) array is used.
For example, a writing optical system which is composed of laser diodes and a rotary polygon scanner is widely used as a writing optical system for use in a laser printer having a low or medium printing speed, such as a printer having a printing speed not higher than 40 PPM (page per minute) in the case where A4/letter size paper is used and the resolution degree is 600 DPI (dot per inch).
But, the recent demands for a higher printing speed and a high-quality printing in half tones cannot be met by the writing optical system composed of laser diodes and a rotary polygon scanner since the switching speed of laser diodes is not sufficiently high and the technology of rotating the rotary polygon scanner at a high speed is insufficient, and this becomes a serious problem. Note that a writing optical system utilizing an LED array is expected to have a high speed since writing is conducted based on parallel exposure system, but there is a problem that luminances of individual LEDs are not uniform.
However, another optical modulator with which these problems may be possibly solved has recently been disclosed for the use in a display apparatus (see the U.S. Pat. No. 5,311,360, and Solid State Sensors and Actuators Workshop, Hilton Head Island, S.C., Jun. 13-16, 1994). This is a micromachine phase diffraction grating utilizing diffraction of light, which is called grating light valve (hereinafter referred to as GLV) element. By utilizing the GLV element, it is possible to electrically control the optical ON/OFF control. In addition, by using the GLV element, a digital optical modulator can be realized which may be substituted for the rotary polygon scanner.
However, no consideration has been made on using the GLV elements disclosed in the above patent specification and other publications as a writing device in an optical printer.